Problem: In geometry class, the girl to boy ratio is $9$ to $6$. If there are a total of $30$ students, how many girls are there?
Answer: A ratio of $9$ girls to $6$ boys means that a set of $15$ students will have $9$ girls and $6$ boys. A class of $30$ students has $2$ sets of $15$ students. Because we know that there are $9$ girls in each set of $15$ students, the class must have $2$ groups of $9$ girls each. There is a total of $18$ girls in geometry class.